A cosmological background in this wave band has been difficult
to unambiguously establish. In the range 10--30 µ there is
a strong source of radiation from heated interplanetary dust
(which is why deep far-infrared surveys are done at the ecliptic poles).
At wavelengths, 30-150
µ heating of the dust
in regions of star formation and/or by the general interstellar
radiation field produces a very strong galactic background even
at relatively high galactic latitude. The IRAS satellite, launched,
in 1984, was sensitive to source emission in the range 12-100 µ.
The all-sky map (Figure 6-1)
produced at 100µ by IRAS clearly shows the presence
of large regions of emission at high galactic latitude. These sources
have been named "Galactic Cirrus" as their structure is fairly
cloud-like. The heating sources of this high latitude cirrus aren't
completely clear but most of the heating probably comes from stars
in the galactic plane, where the optical and UV radiation can escape
through regions of low opacity and penetrate to high latitude. The
typical dust temperature of a cirrus structure is 20-30K. IRAS
observations of nearby large galaxies (e.g., M31, M33) have shown that
this cirrus component is ubiquitous (see Walterbos and Schwering 1987).

Since heated dust has been determined to be pervasive in galaxies,
then the aggregate of all the galaxies in the Universe
should produce a redshift-smeared background
over the range 10-400 µ. In general, the spectrum of dust
emission is that of a blackbody convolved with the emissivity of the
dust grains. The total energy emitted by the dust scales as
Tdustn+4 where n is the emissivity index,
n.
Large grains, which dominate the emission at long
wavelengths, have an emissivity which goes as
-1 and
hence the total energy goes as T5. This is an important point
of energy conservation and balance in galaxies. Small differences
in dust temperature between galaxies or between regions in the
same galaxy, reflect very large differences in energy input.
Simple modeling of the interaction between the radiation field
and the dust temperature (see Bothun, Lonsdale and Rice 1989)
suggests that the dust temperature is a good diagnostic of the nature
of the heating sources (e.g., UV radiation from newly formed stars vs.
the ambient light from older stars). Since the extragalactic background
represents the sum of sources at different wavelengths, then it
will not be characterized by a simple blackbody of some given
temperature. However, its possible that there might be a "feature"
in the spectrum that would represent high star formation rates at
high redshift. This, of course, assumes that dust, which must come
from previous generations of massive star formation, is already in
place in these galaxies at high redshift.

A detection of a possible
cosmological infrared background (hereafter the CIB), means detecting
an isotropic signal, of unknown spectral signature, against
the strong signal of our Galaxy, which, due to high latitude cirrus
emission, is nearly isotropic itself. Further difficulty arises
when trying to calibrate the absolute strength of the CIB as all the
strong foreground sources need to be properly removed. To unambiguously
detect the CIB then requires very good modeling of the the known
solar system and Galactic foregrounds to examine if there are significant
sources of residual emission that are isotropically distributed. Attempts
to do this with the IRAS data did not yield any strong results. However,
one of the instruments on board COBE was the Diffuse Infrared Background
Experiment (DIRBE) which made measurements over the range 1.25-240
µ
(significantly longer than IRAS). The DIRBE measurements are potentially
quite sensitive to the existence of the CIB.

Estimated minimum strengths of this background can be obtained by
using the existing far-infrared (FIR)
Luminosity function (LF) of galaxies, as determined
by IRAS. Over the range 100-300 µ, these minimum strengths are
in the range 2-4 x 10-9 Watts m-2 sr-1
when the FIR LF
is integrated out to z = 3. Any luminosity evolution in the sources
and/or increase in space density with redshift would make the
real background potentially much higher than this minimum estimate.
The current status of the modeling, coupled with the DIRBE
observations, shows residuals of 20-50 x 10-9 Watts m-2
sr-1
at high latitude in this wavelength range.

While this is a possible
detection of the CIB, at a level at least 10 times the predicted
minimum strength, it could equally as well be an indication that
the current modeling of foreground sources is inadequate. Further,
FIR missions, such as ISO and SIRTF will help to better determine
the CIB if it exists. Moreover, even if the CIB were unambiguously
established its origin would still be difficult to interpret. The
low angular resolution at FIR means that many discrete sources (e.g.,
galaxies) would fill a single beam. The expectation, of course, is
that the CIB is produced by galaxies, but at high redshift, intergalactic
dust heated by QSOs could also produce a CIB (see Heisler and Ostriker
1988).